Function Of Laser Pulse Width Research Articles

Collisionless phenomena in heteronuclear clusters

Argon K-shell x-ray yields and electron fluxes are measured for water doped Ar clusters as a function of laser pulse width. The high energy electron and x-ray yields are significantly enhanced in doped clusters. We show that doping of the clusters can be used to enhance collisionless absorption.

Laser-Induced Reshaping of Metallodielectric Nanoshells under Femtosecond and Nanosecond Plasmon Resonant Illumination

Absorption of pulsed laser light energy at the plasmon resonant wavelength of silica−gold core−shell nanoparticles results in a reshaping of the nanoparticle's metal shell layer. Dramatically distinct and reproducible changes in nanoparticle morphology occur as a function of laser pulse width in the femtosecond versus nanosecond regime and laser fluence. The changes are accompanied by large modifications in the plasmon resonant wavelength and spectrum of the reshaped nanoparticle. At the highest femtosecond pulse energies used, deformation of the silica core is also observed.

Transverse emittance measurements from a photocathode RF gun with variable laser pulse length

The gun test facility (GTF) at SSRL was started in 1996 to develop an appropriate injector for the proposed linac coherent light source (LCLS) at SLAC. The LCLS design requires the injector to produce a beam with at least 1 nC of charge in a 10 ps or shorter pulse with no greater than 1π mm mrad normalized rms emittance. The photoinjector at the GTF is 1.6 cell S-band symmetrized gun and emittance compensation solenoid. Emittance measurements, reported here, were made as function of laser pulse width using Gaussian longitudinal pulses. The lowest achieved emittance to date with 1 nC of charge is 5.6π mm mrad and was obtained with a pulse width of 5 ps (FWHM) and is in agreement with simulation. There are indications that the accelerator settings for these results may not have been optimal. Simulations also indicate that a normalized emittance meeting the LCLS requirement can be obtained using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which combined with transverse clipping of the laser is expected to lower the emittance to approximately 1π mm mrad for 1 nC with optimal accelerator settings.

Ionization and Dissociation of Benzaldehyde Using Short Intense Laser Pulses

In a recent series of experiments, femtosecond laser mass spectrometry (FLMS) was applied to benzaldehyde utilizing laser pulse widths in the range 90 fs to 2.7 ps. Beam intensities up to 2 × 1014 W cm-2 were used with wavelengths of 750 and 375 nm. Different ionization−dissociation channels were found compared with previous studies by other authors based in the nanosecond regime. The general theme emerging is one of predominant above-threshold ionization−dissociation (ID) in which predissociative states are largely bypassed via rapid optical up-pumping to the molecular ionization continuum. Above the parent ionization threshold, ladder-switching is seen to be a function of laser pulse width, intensity, and wavelength with exclusive parent ion formation being achievable in the lower intensity regions at all pulse widths. Increasing fragmentation occurs as the laser intensity increases, although parent supremacy remains. At 750 nm however, the increase of fragmentation with intensity is greatly reduced compared to 375 nm, leading to the conclusion that FLMS at longer wavelengths is preferred for chemical analysis. Moreover at 750 nm and at laser intensities close to 1014 W cm-2 C7H6O2+ becomes evident. It is also shown that the C7H6O+/C7H5O+ ratio is strongly dependent on the pulse width, suggesting that a hydrogen loss pathway has a dissociation time of about a picosecond.

Generation of coherent, femtosecond, X-ray pulses in the "water window"

We report experimental and theoretical results on high-harmonic generation with 25-fs laser excitation pulses. The shortest wavelength we observe, at 2.7 nm, is well within the "water window" region of X-ray transmission. In the case of all the noble gases, we obtain excellent agreement between theoretical predictions for the highest harmonic photon energy generated and our experimental observations. We also obtain excellent agreement between theory and experiment for the highest photon energy generated as a function of laser pulsewidth between 25 and 100 fs. Finally, we observe that the individual harmonic peaks near the cutoff are well resolved for positively chirped pump pulses, but are unresolved in the case of negatively chirped excitation pulses.

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF2 are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.

Non-Markovian optical dephasing dynamics in room temperature liquids investigated by femtosecond transient absorption spectroscopy: Theory and experiment

This paper examines the electronic dephasing dynamics of the dye molecule HITCI in an ethylene glycol solution. Degenerate transient absorption data are presented as a function of laser pulse width and detuning from the absorption maximum. The dephasing dynamics revealed by the experimental results are shown to be non-Markovian. The experimental data cannot be quantitatively described by stochastic theories that assume a single relaxation time for the solvent. A model is presented that includes contributions from both fast and slow modulations to the absorption line shape. Using this approach, quantitative agreement is observed between theory and experiment. These results clearly indicate that multiple time scales for liquid dynamics contribute to the absorption line shape of solute molecules. The solvent parameters obtained are discussed in light of results from complementary experiments designed to measure characteristic relaxation times.